diff --git a/custom_components/houseplan/geometry_migration.py b/custom_components/houseplan/geometry_migration.py
index b4281cb..48e8d97 100644
--- a/custom_components/houseplan/geometry_migration.py
+++ b/custom_components/houseplan/geometry_migration.py
@@ -82,12 +82,12 @@ def migrate_space(space: dict[str, Any]) -> bool:
         if shape.get("h") is not None:
             shape["h"] = float(shape["h"]) * ky
 
-    vb = space.get("view_box")
-    if isinstance(vb, list) and len(vb) == 4:
-        space["view_box"] = [
-            dx + float(vb[0]) * kx, dy + float(vb[1]) * ky,
-            float(vb[2]) * kx, float(vb[3]) * ky,
-        ]
+    # The viewport becomes the whole square rather than the transformed old
+    # rectangle. It is what the grid is drawn over and what "fit to screen"
+    # fits, so keeping the old box would leave the new margins outside the
+    # canvas — no dots, nothing to draw on — which is exactly the room this
+    # change was meant to give.
+    space["view_box"] = [0.0, 0.0, 1.0, 1.0]
 
     # The grid pitch is a fraction of the WIDTH. A tall plan just got a wider
     # canvas, so a wall now covers fewer cells; without this every measurement
diff --git a/tests_backend/test_validation.py b/tests_backend/test_validation.py
index 6d83b21..38e5b4b 100644
--- a/tests_backend/test_validation.py
+++ b/tests_backend/test_validation.py
@@ -688,6 +688,17 @@ def _sq(space, layout=None):
     return cfg["spaces"][0]
 
 
+def test_the_viewport_becomes_the_whole_square():
+    """The grid is drawn over the view box, and the fit fits it.
+
+    Transforming the old rectangle instead would leave the new margins outside
+    the canvas — no grid there and nothing to draw on — which is the room the
+    square canvas was meant to add.
+    """
+    sp = _sq({"id": "f1", "aspect": 0.5, "view_box": [0.1, 0.2, 0.5, 0.5], "rooms": []})
+    assert sp["view_box"] == [0.0, 0.0, 1.0, 1.0]
+
+
 def test_a_wide_plan_gains_margins_above_and_below():
     sp = _sq({
         "id": "f1", "aspect": 2.0, "cell_cm": 5, "view_box": [0, 0, 1, 1],
@@ -718,7 +729,7 @@ def test_a_square_plan_is_left_alone():
     }
     sp = _sq({**before, "rooms": [dict(before["rooms"][0])]})
     assert sp["rooms"][0] == before["rooms"][0]
-    assert sp["cell_cm"] == 5 and sp["view_box"] == [0, 0, 1, 1]
+    assert sp["cell_cm"] == 5 and sp["view_box"] == [0.0, 0.0, 1.0, 1.0]
 
 
 def test_migration_preserves_real_lengths_and_shapes():